The present invention relates to compression release mechanisms for internal combustion engines which operate a valve at slow speeds to release pressure within the engine cylinder during the compression portion of the combustion cycle.
It is desirable in internal combustion engines to reduce the force required to turn over the engine during starting. It is particularly advantageous to minimize starting forces in small internal combustion engines which are to be started by hand. In addition, such hand started engines must provide a mechanism to eliminate the danger of physical injury from engine kickback commonly caused by force from engine compression.
The chief cause of difficulty in turning over an internal combustion engine is the compression within the cylinder during a portion of the engine cycle. The prior art is replete with mechanisms to reduce that compression during starting. Early devices provided a manually operated valve which released the pressure from the cylinder. The disadvantage of a manual valve is that it must be quickly closed by the operator after cranking in order for the engine to start. Thus manually operated valves require a certain amount of skill in order to properly start the engine and are susceptible to operator oversight.
The prior art teaches a variety of automatic compression release mechanisms which are governed by the speed of the engine. At low speeds, the compression release mechanism opens a valve during the compression portion of the combustion cycle. When the speed increases above a given amount, the mechanism closes the valve enabling normal engine compression to occur.
Many automatic devices utilize the exhaust valve for the engine cylinder to release the compression while starting the engine. In this type of device, the compression release mechanism operated in conjunction with the camshaft on which a lifter for the exhaust valve rode. An example of such a mechanism is described in U.S. Pat. No. 4,892,068. This compression release device has a crescent shaped flyweight pivotally mounted to a gear on the camshaft and the flyweight rotates a cam pin into different positions depending upon engine speed. In one position, occurring at slow speeds, the cam pin raises an exhaust valve lifter from the surface of the camshaft during engine compression. After the engine starts, the greater speed causes the flyweight to rotate the cam pin into another position at which the pin no longer engages the exhaust valve lifter, allowing the engine to operate normally.